Get valid mashlib html file location

In the latest npm, node_modules is flattened, so it shouldn't be located based on the location of solid-server.

This solves #1382

Author: linonetwo

lib/handlers/get.js

@@ -89,7 +89,7 @@ async function handler (req, res, next) {
 
     if (useDataBrowser) {
       res.set('Content-Type', 'text/html')
-      const defaultDataBrowser = _path.join(__dirname, '../../node_modules/mashlib/dist/index.html')
+      const defaultDataBrowser = require.resolve('mashlib/dist/index.html')
       const dataBrowserPath = ldp.dataBrowserPath === 'default' ? defaultDataBrowser : ldp.dataBrowserPath
       debug('   sending data browser file: ' + dataBrowserPath)
       res.sendFile(dataBrowserPath)